My U. S. Pat. Nos. 3,735,752; 3,947,739; 4,090,406; 4,163,390; 4,259,968 and my U.S. Pat. Application Serial No. 236,546, filed August 25, 1988, which relate generally to spirometers, flowmeters and related sensitive fluid flow measuring systems are fully incorporated herein by this reference.
My U. S. Pat. Nos. 4,163,390 and 4,259,968 describe a spirometer having a breath transmission passage with opposite ends open to the atmosphere. A restriction inside the passage produces a pressure drop in the passage in response to inhalation or exhalation by a patient into the end of the passage. The restriction is formed by an elongated, small-diameter tube extending along the main axis of the breath transmission passage. A pair of flow measurement passages in the breath transmission passage adjacent the restriction are coupled to a flowmeter which measures the pressure drop near the restriction. This provides a measurement of gas flow rate in the breath transmission passage.
When the breath transmission passage has a large diameter (approximately one inch diameter or more), and when the gas flow rate through the passage is low and the gas flow enters the passage from a corrugated tube attached to the passage, no output signal is developed by the flowmeter because the pressure drop across the flow measurement passages is too low. Use of a largediameter tube for the breath transmission passage of a spirometer is desirable; but when the diameter is large and the gas flow rate is low, the gas tends to flow toward the center of the passage. Therefore, there is no pressure drop near the outside of the passage, including the region connected to the flowmeter. It is a standard practice in the use of spirometers to use a flexible corrugated tube attached to the spirometer breath transmission passage. The flexibility of the corrugated tube is a desirable attribute during use, but gas flow into the breath transmission passage is not laminar for low gas flow rates because the corrugations in the tubing create small eddy currents near the outer wall of the passage, near its connection to the corrugated tubing. The loss of a more laminar and uniform gas flow prevents the necessary pressure drop near the outside of the breath transmission passage and results in inaccurate readings or no signal at all at the flowmeter.
One technique for producing a more laminar flow through the breath transmission passage is to place a screen across the cross section of the passage. However, the screen needs a support, and this creates turbulent flow, especially at higher gas flow rates. Another serious drawback is that the screen can be clogged from the patient's coughing.
The present invention provides a multiple tube fluid flow measuring device useful in a spirometer for producing accurate flowmeter signals for gas flow through a breath transmission passage having a large diameter, when the gas flow rate through the passage is low, and when the gas flow enters the passage from a corrugated tubing. The multiple tube fluid measuring device makes gas flow more laminar through the cross section of the passage and produces a measurable pressure drop near the flow measuring passages leading to the flowmeter, even for low gas flow rates. The low flow rate sensitivity of the measuring device is increased substantially over prior art measuring devices, while producing a negligible increase in gas flow resistance through the breath transmission passage.